A PRELIMINARY TRANSMISSION ELECTRON MICROSCOPY INVESTIGATION OF COMPLEX EXCHANGE AND INTERACTIONS OF SELECTED RARE EARTH ELEMENTS WITH SYNTHETIC CRYPTOMELANE

Cryptomelane is a manganese oxide common to rock varnish and soils that is used as an octahedral molecular sieve (OMS-2) material. Relatively little is known regarding the sorption and interaction of rare earth elements (REE) by cryptomelane with most work involving very basic catalytic studies of Ce-cryptomelane produced synthetically. REE exchange and interactions with cryptomelane may be extremely important for understanding the controls of distribution of REE in soils, rock varnish, ores and non-traditional sources of REEs. Previous investigations have shown that Mn-rich coatings such as rock varnish may have comparatively high REE content.

We investigated the exchange of selected REEs with a synthetic cryptomelane to determine if they could readily exchange for K in the tunnel site. Synthetic cryptomelane was prepared with a standard common sol-gel synthesis method. Selected REEs were exchanged with cryptomelane using nitrate solutions. Reaction produced both exchange and apparent mineralization of oxide phases enriched in REEs.

Transmission electron microscopy (TEM) investigation indicates extensive exchange of K with some REEs. We observed multiple 5-7nm euhedral crystalline phases, enriched in REEs, associated with residual cryptomelane fibers, which display apparent corroded textures. This implies a dissolution-reprecipitation process rather than exchange. The starting morphologies of the synthetic cryptomelane were largely euhedral fibers and typically several microns in length and 60 to 120 nm in width. Selected area electron diffraction shows common diffraction spots at 0.73 nm and other common diffraction spots were found at 0.14 nm. Lattice fringe images obtained are consistent with octahedral widths with a spacing of approximately 0.26 nm. Spot energy dispersive spectroscopy (EDS) shows that some particle aggregates are enriched in REEs by several weight percent (numbers?).

Our preliminary results demonstrate that cryptomelane influences the distribution of REEs in earth surface environments by both exchange and mineralization of REE oxides on cryptomelane surfaces. The specific mechanism of exchange and mineralization is unclear, but our results suggest further detailed investigations are warranted.